This invention relates to hot charge-carrier transistors having a base region through which current flow is by hot majority charge-carriers, and relates particularly but not exclusively to hot electron transistors formed with monocrystalline gallium arsenide.
U.S. Pat. No. 4,149,174 and published United Kingdom patent application (GB-A) No. 2 056 165 disclose hot charge-carrier transistors comprising a base region through which current flow is by hot majority charge-carriers of one conductivity type. Barrier forming means forms with the base region an emitter-base barrier serving for injection of the hot charge-carriers of said one conductivity type into the base region. There is a semiconductor collector region of said one conductivity type. The base-collector barrier in these transistors is formed by a semiconductor barrier region which is doped with impurity of the opposite conductivity type and which is sufficiently narrow as to form with said semiconductor collector region a bulk unipolar diode for collecting the hot charge-carriers of said one conductivity type from the base region during operation of the transistor.
Related devices are shown in commonly-owned U.S. Pat. Nos. 4,797,722 and 4,843,447.
In order to obtain a high current gain from such a transistor, the hot charge-carriers injected into the base region should have a high energy compared with the barrier height of the base-collector barrier. For this reason it is usually desirable to make the energy difference between the barrier heights of the emitter-base and base-collector barriers as large as possible.
However the present invention is based on a recognition by the present inventor that, as the height of the emitter-base barrier approaches the bandgap of the semiconductor material of the collector region, some of the hot charge-carriers which pass into the collector region can create electron-hole pairs by ionization (particularly with a large base-collector bias voltage applied); that the minority carriers thus generated can reduce the speed of the transistor by being stored in the base-collector barrier region (and possibly emitter-base barrier region); and that such ionization can be reduced (and even substantially eliminated) by cooling the hot charge-carriers in the collector region by means of a retarding electric field at a heterojunction associated with the collector region.